There are many systems in the prior art for precisely measuring the displacement of an object. These include, for example, optical scale system, magnetic scale systems, fiber optic systems, interferometer systems, and other types of systems and apparatus. For example, F. Parmigiani describes in Optical and Quantum Electronics, 10, pages 533-535 (1978), a fiber-optic probe apparatus which is used to measure small displacements. F. Bien, M. Camac, H. J. Caulfield and S. Ezekiel, describe in Applied Optics, 20, pages 400-403 (1981), a system which uses variable wavelength interferometer techniques to measure absolute distances. A. Olsson and C. L. Tang, describe in Applied Optics, 20, pages 3503-3507 (1981), a system using dynamic interferometry techniques to measure small optical path length changes. C. P. Wang, R. L. Varwig and P. J. Ackman describe in the Review of Scientific Instruments, 53,, pages 963-966 (1982), a system using acousto-optical techniques to measure mirror displacements of extremely short ranges. J. Berger and R. H. Lovberg described in Science, 107, pages 296-303 (1970), apparatus using a laser interferometer to measure earth strain.
There are also commercial displacement measuring devices available on the market. These include, for example, the Laser Position Transducer, Model HP5527A, manufactured by Hewlett-Packard, Palo Alto, Calif.; the Interferometric Position Sensor, Model TIPS-IV, manufactured by Telectrac, Goleta, Calif.; the Interferometer System III, Model 7910, manufactured by Mark-Tech, San Jose, Calif.; an Optical Encoder Gauge, manufactured by Bausch & Lomb, Rochester, N.Y.; and a Magnetic Encoder Gauge, manufactured by Sony of Japan. In general, the devises using interferometric techniques are more accurate than those using optical encoding techniques, but alignment is more critical in the former, and therefore they are more expensive.
It is also usual practice in the prior art to utilize Doppler radar principles for measuring the velocity of a moving target. Typical apparatus is described, for example, in a text by M. I. Skolnik, entitled "Introduction to Radar Systems", Chapter 3, McGraw-Hill, New York (1980); and in an article entitled "Laser Doppler Velocimeter", by C. P. Wang, American Scientist, 65, Pages 289-293 (1977).
Most prior art precision displacement measuring devices use interferometer techniques, and such prior art devices usually include an expensive, sophisticated frequency stabilized laser to provide measurements. Accordingly, the prior art precision displacement measuring apparatus is extremely critical insofar as alignment is concerned, and it is very expensive. The apparatus of the present invention, on the other hand, is of the electro-optical type, and it uses the Doppler shift of a laser frequency caused by a moving target accurately to measure its displacement over several meters.
Specifically, the apparatus of the present invention is based on radar principles, Doppler effects and optical heterodyning. The apparatus of the invention is similar in some respects to a Doppler radar system, in that the target in the system of the invention is a moving retro-reflector. However, in the apparatus of the invention the target is illuminated by a laser beam. The laser beam reflected by the retro-reflector target is frequency shifted by the movement of the target. The Doppler frequency shift of the reflected laser beam is proportional to the velocity of the target, and the phase shift of the reflected laser beam is proportional to the displacement of the target from a reference position.
The system and apparatus of the invention includes a phase detector which is used to detect the phase shift of the reflected laser beam which, as mentioned above, represents the displacement of the retro-reflector target from a reference position. When the displacement of the target is greater than a half-wavelength, a counter may be used in the apparatus to determine the total phase changes. The apparatus also includes a microprocessor which reads the count of the counter, and the phase angle of the reflected laser beam, and which functions to convert the readings into inches or centimeters.
Briefly stated, the apparatus of the present invention is advantageous as compared with the prior art apparatus, in that it is compact, simple to use, requires no precise fittings or critical alignments, and it exhibits drift-free accuracy.
The apparatus and system of the invention is predicated upon unique optical heterodyne and optical modem principles which make for practical, economical, versatile, accurate, and convenient displacement measurements. Furthermore, the apparatus of the invention is advantageous in that it can use readily available inexpensive lasers. Moreover, the apparatus of the invention is fast acting, in that it exhibits a slew rate higher than 50 m/minutes in a constructed embodiment. The apparatus of the invention is simple in its construction in that it does not require an interferometer, and it is easy to align since it possesses large misalignment tolerances of the order of 30 seconds in angle and 1 millimeter in position.
Specifically, the advantages of the system and apparatus of the present invention include the following:
A. There is no requirement for calibration, since measurements are made using the speed of light and laser frequency;
B. There are no critical alignment or precision fitting requirements;
C. The retro-reflector target does not require an interferometer module;
D. The apparatus does not exhibit any wear, backlash, or periodic errors; and
E. The apparatus uses available low-cost laser as its light source.